Fast-neutron nuclear reactors of the integrated type comprise a vessel of very large dimensions, called a main vessel, which contains the primary cooling fluid of the reactor, generally consisting of liquid sodium in which are immersed the core and the internal structures of the reactor as well as the lower part of the components of the reactor consisting of heat exchangers and pumps which penetrate into the vessel through a slab closing the upper part of the vessel.
The main vessel of the reactor comprises a wall consisting of stainless steel plates shaped and assembled by butt welding along lengthy weld lines.
The main vessel is disposed inside a second vessel, called the safety vessel, so as to form with the safety vessel an inter-vessel space of substantially constant width.
Because of the presence of liquid sodium which is capable of spontaneously igniting in contact with an oxidizing atmosphere, the sodium mass is surrounded by a neutral gas atmosphere, generally consisting of argon which fills, in particular, the inter-vessel space.
Although the manufacture of the vessel of the nuclear reactor is subject to very specific precautions and is accompanied by numerous examinations at all stages of the work, the vessel may include certain defects which do not compromise safety because the probability of their further development is substantially zero. Furthermore, these defects, such as a local deficiency in thickness of a plate forming part of the wall of the vessel, or a deficiency in compactness of a weld, are known because, they have been detected and accepted during manufacture.
In order to detect a highly improbable development of these defects after a certain duration of operation of the reactor, a non-destructive examination is made of the corresponding zones of the walls, at regular intervals, during shutdowns of the nuclear reactor.
Furthermore, since certain defects such as cracks can appear in the wall of the vessel during operation of the reactor, it is necessary to detect such defects as quickly as possible after their appearance.
Various known non-destructive examination techniques, such as radiography, magnetic-particle examination techniques, liquid penetrant examination and ultrasonic or eddycurrent examination methods can be used in order to examine the qualitative state and the integrity of an element of an industrial assembly such as a nuclear reactor.
However, such methods may prove difficult if not impossible for use in examining the wall of the main vessel of a fast-neutron nuclear reactor. The interior of the vessel may not actually be accessed even during periods of shutdown of the reactor, since this vessel contains the core of the reactor which has very high activity and which is filled with hot and radioactive liquid metal.
After the reactor has been put into service, the wall of the main vessel may only be inspected from outside the vessel, the operation having to be remote controlled because of the presence of a neutral gas atmosphere around the vessel, because of the high temperature the vessel which contains hot liquid sodium, even during periods of shutdown of the reactor, and because of intense gamma radiation.
It has been proposed, for conducting the inservice inspection of the wall of the main vessel of a fast-neutron nuclear reactor, to move the inspection means in the inter-vessel space, in the vicinity of the external surface of the main vessel, using a hinged trolley whose movements are remote controlled. However, inspection means which can be associated with the trolley in order simply to conduct the examination of zone or the whole of the main vessel of the nuclear reactor are not known.
Non-destructive examination methods using rays, i.e., photon fluxes of certain energy emitted by a radioactive substance, in the form of a point source, are known.
The photons are capable of passing through a certain thickness of a material to be examined and photon flux passing through the material is modified by the presence of defects. By making a count or a measurement of photon fluxes after passage through the material, it is possible to detect the possible presence of defects in the material.
If a wall is being examined, it is necessary to dispose a gamma-ray emitter and a photon detector on either side of such wall, in corresponding positions. Such a method is therefore not applicable in the case of a main vessel of a fast-neutron nuclear reactor, since it is not possible to place an examination means inside the vessel.
Equally, in the case of a vessel of a pressurized-water nuclear reactor, it is not possible to access the internal volume of the vessel during operation of the reactor, with the result that it is not possible to conduct an inspection of the wall of the vessel in service, using gamma rays emitted by a point source, according to prior art.
In the case of nuclear reactors comprising a vessel containing a liquid for cooling the reactor in which the core is immersed, the cooling liquid may be activated during operation of the reactor, for example under the effect of neutron bombardment.
In the case in which the cooling liquid is sodium, the neutron bombardment coming from the core leads to the formation of two radioactive isotones of sodium, namely sodium-22 and sodium-24. These two elements, which are distributed in the whole of the mass of the liquid sodium filling the vessel, are gamma-ray emitters.
In the case of a nuclear reactor cooled by pressurized water, nuclear reactions occur during the operation of the reactor which lead to the formation of radioactive nitrogen-16 distributed throughout the mass of the primary water filling the vessel. The nitrogen-16 continuously formed in the vessel of the reactor in operation emits gamma rays because of its radioactivity.
Receptacles or tanks which are intended to contain liquids containing one or more gamma-ray emitter radioactive elements distributed in the mass of the liquid contained in the tank are more generally used in the nuclear industry.
A radioactive element contained and distributed in the mass of a liquid contained by a tank has to date never been used in order to perform the non-destructive examination of the wall of the tank.